Low rates of nitrogen fixation in eastern tropical South Pacific surface waters

An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N-2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N-2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N-2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 mu mol N m(-2) d(-1) based on sediment trap fluxes. Whereas the fractional importance of N-2 fixation did not change, the N-2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N-2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N-2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N-2-fixation rates were higher, up to 100 mu mol N m(-2) d(-1), and detected N-2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N-2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N-2 fixation in the ETSP.